WASHINGTON — Democrats maneuvered a massive overhaul of the nation's health care system toward a make-or-break vote in the House on Saturday, getting a last-minute appeal from President Barack Obama to pass his top domestic priority.

"This is our moment to deliver," Obama said.

Emerging from a closed-door meeting with the president, Speaker Nancy Pelosi predicted the bill will pass. "We can associate ourselves with the work of those who passed Social Security, those who passed Medicare," she said.

Meeting with Democrats in a rare Saturday visit to Capitol Hill, Obama said they were approaching what may be their finest moment in politics.

Later, in public remarks in the White House Rose Garden, Obama said, "What's in our grasp right now is a chance to prevent a future where every day, 14,000 Americans continue to lose their health insurance, and every year, 18,000 Americans die because they don't have it."

The bill survived a test vote with a 50-vote margin in a 242-192 roll call, but the vote on final passage was expected to be much closer. Fifteen Democrats joined all 177 Republicans in voting to block the debate.

"This bill will force my constituents to buy insurance whether it makes sense for them or not," said Rep. Cynthia Lummis, R-Wyo.

Obama made his trip to the Capitol complex as abortion rights lawmakers voiced anger at a last-minute concession granted to foes of the procedure, who were given a vote on their proposal for stronger restrictions on abortion coverage.

"There is a risk" that some in the Pro-Choice Caucus would vote against the legislation if the stricter curbs are adopted, said Rep. Diana DeGette, D-Colo.

The bill would cost $1.2 trillion over the next decade. It would provide health coverage to tens of millions of Americans who don't have it now, require most employers to offer it to their workers and prohibit insurance companies from denying coverage based on a person's medical history.

"The status quo is unaffordable and unsustainable. Health care reform benefits all of us," said Rep. Allyson Schwartz, D-Pa., as debate opened on the House floor.

House passage of the bill is crucial if Obama is to fulfill the biggest promise of his campaign last year. But the legislation still faces multiple hurdles, and a Senate vote on it might not occur until next year.

Republicans were united in their opposition.

"The American people need to understand this is about a government takeover of the whole health care system," said Rep. Paul Broun, R-Ga.

The most contentious issue is a new government-run insurance plan that would be offered alongside private coverage within new purchasing marketplaces, or "exchanges," where individuals and small businesses could shop for and compare options.

The abortion agreement was reached at midnight Friday after hours of intense negotiations brokered by Pelosi, D-Calif. Democratic Reps. Bart Stupak of Michigan, Brad Ellsworth of Indiana and other abortion opponents won an opportunity to insert tougher restrictions into the legislation during debate, despite fervent opposition from abortion-rights liberals who are a driving force behind the overall bill.

"We wish to maintain current law, which says no public funding for abortion," Stupak said.

Federal law currently prohibits the use of federal funds to pay for abortions except in cases of rape, incest or situations in which the life of the mother is in danger. Left unresolved is whether individuals would be permitted to use their own funds to buy insurance coverage for the procedure in the federally backed insurance exchange envisioned under the legislation.

Stupak's amendment would deny abortion coverage to anyone who gets federal insurance subsidies or buys a policy from the government, except in cases of rape or incest or if the mother's life is in danger. People could buy separate policies covering just abortions using their own money.

DeGette called Stupak's amendment "the biggest restriction on a women's right to chose that's been considered on the floor of the House" in her 13 years in office.

The leadership's hope is that no matter how the vote on the abortion measure turns out, Democrats on both sides of the abortion divide will then unite to give the health care bill a majority over unanimous Republican opposition.

With Democrats' command of the necessary votes looking tenuous, Obama threw the weight of his administration behind the effort to round up support. Prior to his trip to the Capitol, he and top administration officials worked the phones to pressure wavering lawmakers.

Undecided Rep. Jason Altmire, D-Pa., said he heard from Obama, White House Chief of Staff Rahm Emanuel, Health and Human Services Secretary Kathleen Sebelius and Education Secretary Arne Duncan.

Democrats hold 258 seats in the House and can afford 40 defections and still wind up with 218, a majority if all lawmakers vote.